Start-up control in power systems using fixed-ratio power conversion

ABSTRACT

A power converter system converts power from an input source for delivery to an active load. An input current surge at startup may be reduced by combining power converter switch resistance modulation with active load control. In another aspect, an input current surge at startup in an array of power converters may be reduced by periodically reconfiguring the array during the startup phase to accumulatively increase the output voltage up to a predetermined output voltage. A power converter may include a controller that provides an over-current signal to the load to reduce the load or advise of potential voltage perturbations.

This application is a continuation of U.S. patent application Ser. No.15/976,773, filed May 10, 2018, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

This invention generally relates to controlling the rate at which energyis transferred from an electrical source to a load during systemstart-up, and more particularly, to control of fixed-ratio powerconverters and loads.

BACKGROUND

Distributing power in electronic systems may include using fixed-ratioconverters, also known as bus converters, to convert a distributed busvoltage to lower or higher voltage required by a specific load. Typicalbus converters may include isolated or non-isolated switching powerconverters which convert power via an inductive component, atransformer, or switched capacitor networks. Frequently, one or moreregulators may be connected to receive power form the bus converteroutput to regulate the power or voltage delivered to respective loads.The loads may include a variety of “active” loads which may present timevarying current demands on the power system. For example, typicalmicroprocessors as loads present significant time varying changes in theload current demanded from the power system. Other electronic circuitryincluding communications circuitry, storage devices, lighting systems,including LED arrays, etc. may similarly present time varying currentdemands on the power system. Such power systems may typically includeenergy storage, such as back-up batteries and hold-up capacitance, andfiltering elements, such as filter capacitances, distributed throughoutthe system, which may be connected directly to the output of the busconverters or further downstream, e.g. at the output of interveningconverters, such as voltage regulators, and directly across the loads.Current surges on system start-up or re-introduction of power maychallenge many elements of the system including, power switchingelements in the bus converters.

SUMMARY

One exemplary embodiment includes an apparatus for delivering power froman input source at a source voltage, V_(S), to a load at a load voltage,V_(L). A bus converter may have a plurality of power switching elementsand a switch controller to operate the power switching elements in aseries of converter operating cycles during start-up and steady-statephases of operation to convert power received from the converter inputat an input voltage, V_(IN), and input current, I_(IN), for delivery tothe converter output at an output voltage, V_(OUT), and an outputcurrent, I_(OUT), using a transformation ratio, K, whereK=V_(OUT)/V_(IN), that is essentially constant. The bus converter mayturn the power switching elements ON in a high resistance state in thestart-up phase to increase the equivalent series resistance and in a lowresistance state in the steady-state phase to decrease the equivalentseries resistance. A control circuit may provide a control signal todisable the load during the start-up phase and enable the load duringthe steady-state phase.

Another exemplary embodiment includes an apparatus for delivering powerfrom an input source at a source voltage, V_(S), to a load at a loadvoltage, V_(L). A bus converter may have a plurality of power switchingelements and a switch controller to operate the power switching elementsin a series of converter operating cycles during which power receivedfrom the converter input at an input voltage, V_(IN), and input current,I_(IN), is converted for delivery to the converter output at an outputvoltage, V_(OUT), and an output current, I_(OUT), via the powerswitching elements during a start-up phase of operation and during asteady-state phase of operation. The bus converter may convert powerusing a transformation ratio, K, where K=V_(OUT)/V_(IN), that isessentially constant in the steady-state phase of operation. The busconverter may limit the current flowing through the power switchingelements to a predetermined in-rush limit in the start-up phase ofoperation. The in-rush limit may be less than the level of currentpermitted during the steady-state phase. A control circuit may provide acontrol signal to disable the load during the start-up phase ofoperation and enable the load during the steady-state phase ofoperation.

Various illustrative implementations of the above embodiments mayinclude one or more of the following features, alone or in combinationwith one another. The bus converter may operate in the start-up phasewhen the output voltage is below a first threshold and in thesteady-state phase when the output voltage is above a second threshold.The bus converter may operate in the start-up phase after the switchcontroller receives power and a converter enable signal. The switchcontroller may adjust a drive level for the power switching elementswithin a range during the start-up phase from an initial value forturning the one or more of the power switching elements ON with ahighest switch resistance to a final value for turning the one or moreof the power switching elements ON with a lowest switch resistance. Theswitch controller may adjust the drive level during the start-up phaseas a function of one or more of the following: input current, outputcurrent, switch current, or a voltage across a resonant capacitor. Thebus converter may transition to the steady-state phase after the drivelevel of the one or more power switching elements reaches the finalvalue. The bus converter may include the control circuit and the switchcontroller may send a signal to the control circuit after the finalvalue is reached. The switch controller may monitor a voltage across aresonant capacitor (e.g., may include an input connected to monitor thevoltage) and provide a signal (e.g., via an output) indicating that acurrent drawn from the converter output meets or exceeds predeterminedcriteria during steady-state operation. The predetermined criteria mayinclude any one or more of the following: a predetermined change in thecurrent, a predetermined rate of change in the current, or apredetermined threshold level of the current.

The apparatus may include one or more energy storage devices and/orloads connected to the bus converter output. In some embodiments, theapparatus may include one or more regulators, each having a regulatorinput connected to receive power from the bus converter output, aregulator output, and regulation circuitry adapted to convert powerreceived from the regulator input for controlled delivery to the loadvia the regulator output. In some embodiments, the regulators may eachfurther include a control input connected to receive the control signaland is adapted to restrict power delivery to the regulator output duringthe start-up phase. Some of the loads may include a regulator whichrestricts power delivery to the regulator output during the start-upphase in response to the control signal. The regulators may be switchingvoltage regulators. A system circuit board may include the loads, theregulators, and a power bus connecting the bus converter output to theenergy storage devices, the regulators, and other loads. One or more ofthe loads may include electronic circuitry, such as integrated circuits.Some electronic circuitry loads may receive power from a respectiveregulator. Either the regulator or the electronic circuitry of the loadmay in response to the control signal restrict the power it draws duringthe start-up phase. The regulators may be switching voltage regulators.The bus converter and one or more regulators may be assembled to asystem circuit board with the regulators disposed at a distance from thebus converter. In some embodiments, one or more of the regulators may bemounted to the system circuit board.

The bus converter may include an inductive component and may use acurrent flowing in an inductive component to charge and dischargecapacitances in the bus converter to reduce a voltage across one or moreone of the power switching elements prior to turning it ON. The busconverter may be packaged together with the control circuit as aself-contained assembly for installation as a unit. The bus convertermay include a transformer driven by the primary power switching elementsand output circuitry receiving power from the transformer. A resonantcircuit may be formed including the transformer and having acharacteristic resonant frequency and period with the primary powerswitching elements connected to drive the resonant circuit and a switchcontroller operating the primary power switching elements in a series ofconverter operating cycles, each characterized by two power transferintervals of essentially equal duration less than the characteristicsresonant period and during which one or more of the primary powerswitching elements is ON and power is transferred from the input to theoutput via the transformer. The bus converter output may be galvanicallyisolated from or connected to the bus converter input. The inputcircuitry may include a winding having a first number of turns and theoutput circuitry may include a winding having a second number of turns,with the bus converter transformation ratio being a function of theratio of the first number of turns to the second number of turns. Theinput circuitry and at least a portion of the output circuitry may beconnected (e.g., galvanically connected) in series across the source forat least a portion of the converter operating cycle, such that anabsolute value of the input voltage, V_(IN), applied to the inputcircuit is approximately equal to the absolute value of the DC sourcevoltage V_(S) minus a number, N, times the absolute value of the outputvoltage, V_(OUT), where N is at least 1. The bus converter may includean inductive component and use a current flowing in the inductivecomponent to charge and discharge capacitances in the bus converter,where a current through the power switching elements is reduced prior toturning ON or OFF. The voltage across the power switching elements maybe reduced to essentially zero volts prior to turn ON. The bus convertertransformation ratio may be greater than 1 for step-up or less than 1for step down. The bus converter comprises a switched capacitorconverter.

The control circuit may sense the output voltage, V_(OUT), (e.g.,through an input) and disable the load using the control signal when thecontrol circuit determines the output voltage is below a predeterminedthreshold. The predetermined threshold may be a function of the inputvoltage, V_(IN). The determining may be a function of the input voltage,V_(IN), or output voltage, V_(OUT), adjusted by the transformation ratioK. The determining may be a function of the difference between the inputvoltage, V_(IN), and output voltage, V_(OUT), one of which is adjustedby the transformation ratio K. The control circuit may include an inputfor determining the input or output current and disable the load usingthe control signal when the control circuit determines the input oroutput current meets predetermined criteria. The switch controller mayinclude a switch drive controller which senses an indication of theinput current, operates the power switching elements in a high-impedancemode during the start-up phase and in a low-impedance mode during thesteady-state phase, and adjusts the switch drive to limit the inputcurrent. The switch drive controller may increase the switch drive levelduring the start-up phase to maintain the input current at a levelapproximately equal to the in-rush limit. The control circuit may sendthe enable signal after the switch drive reaches a predetermined level,which may be the switch drive level for operation of the switches in thesteady-state phase. The bus converter may include the control circuit.

Another exemplary embodiment includes an apparatus for delivering powerfrom an input source at a source voltage, V_(S), to a load at a loadvoltage, V_(L). A bus converter may have a converter input, a converteroutput, a plurality of power switching elements and a switch controlleradapted to operate the power switching elements in a series of converteroperating cycles to convert power received from the converter input atan input voltage, V_(IN), for delivery to the converter output at anoutput voltage, V_(OUT). A controller may monitor a voltage across aresonant capacitor and provide a signal indicating that the currentdrawn from the converter output meets or exceeds predetermined criteria.

Various illustrative implementations of the above embodiment may includeone or more of the following features, alone or in combination with oneanother. The predetermined criteria may include a predetermined changein the current, a predetermined rate of change in the current, or apredetermined threshold level of the current. The one or more of thepower switching elements may be turned ON in a low resistance state inthe steady-state phase to control the equivalent series resistance ofthe bus converter. The controller may monitor the output current duringthe steady-state phase and provide the signal to the load. The busconverter may be operated in a start-up phase to turn one or more of thepower switching elements ON in a high resistance state to increase theequivalent series resistance of the bus converter. The controller mayprovide a control signal to disable the load during the start-up phaseand to enabled the load during the steady-state phase. The bus convertermay convert power using a transformation ratio, K, whereK=V_(OUT)/V_(IN), that is essentially constant during the steady-statephase. The bus converter may convert power via a transformer. The busconverter may convert power via switched capacitors.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a power conversion system according tothe invention including a fixed-ratio converter and a plurality ofindividual load elements.

FIGS. 2A through 2F show waveforms of a power conversion system during astartup phase and during steady-state operation.

FIG. 3 shows a large signal model of the bus converter.

FIG. 4 shows a bus converter for use in the system of FIG. 1.

FIG. 5 shows details of a start-up phase of the system of FIG. 1.

FIG. 6 shows alternate embodiment of the bus converter of FIG. 4.

DETAILED DESCRIPTION

Referring to the example of FIG. 1, a power system 10 as shown includesa fixed-ratio converter, also known as a bus converter, 60, connected tosupply power to a plurality of loads, e.g. loads 81, 82, 83, 84, and 85.Loads 84 and 85 as shown each include a regulator 84A, 85A, connected toreceive power from the bus converter 60 and regulate the power deliveredto its respective load 84B, 85B, and a filter capacitance 84C, 85Cconnected to the output of its respective regulator. Regulators 84A and85A may preferably use a non-isolated switching voltage regulation, e.g.a buck, boost, or buck-boost, power conversion topology, or othersuitable circuitry, such as battery charge controller, or backlightcontroller, etc. The regulators 84A and 85A may, in some embodiments,each include a regulator input connected to receive power from an outputof the bus converter 60, a regulator output, and regulation circuitryadapted to convert power received from the regulator input forcontrolled delivery to the loads 84B and 85B. The power system 10 mayinclude an electrical power source 50 connected supply power to the busconverter 60 and capacitive energy storage and filtering devices, suchas capacitors 71, 72, 73, 74, 75, connected to the output of the busconverter 60 as shown in FIG. 1. As shown, the energy storage andfiltering devices may be distributed throughout the power system 10,including within, and external to, the bus converter 60. The powersource 50 may as shown in FIG. 1 be a DC source, such as a front endrectification and/or filtering circuit, a power factor correctingconverter, a regulator, or other power source, such as a battery or a 48VDC telecommunications supply.

The bus converter 60 may be generally characterized as operating with anessentially fixed conversion ratio, K=V_(OUT)/V_(IN). The transformationratio may be described as essentially fixed because the finite outputresistance of the converter causes some droop as a function of outputcurrent. For example, the bus converter output voltage may be expressedin terms of the input voltage, transformation ratio, and the outputcurrent: V_(OUT)=K*V_(IN)−I_(OUT)*Rout as discussed below in connectionwith FIG. 3. The output voltage, V_(OUT), may be set to any leveldesired using different conversion ratios, for example, assuming thesource 50 supplies 48 VDC, typical bus converter transformation ratiossuch as K=1/4 may be used to supply 12 VDC, K=1/5 to supply 9.6 VDC,etc., as required by the loads. Multiple bus converters having differenttransformation ratios may be connected to the source 50 to providemultiple output voltages as needed. Although many telecom and computingsystems use bus converters having transformation ratios that are lessthan 1 (K=1/4 down conversion) to supply most loads, the emergence ofASICs and processor cards or packages requiring higher voltages, e.g.48V, may create the need for bus converters having transformation ratiosgreater than 1 (K=4 up conversion) in systems using lower bus voltages,e.g. 12V legacy systems.

Each load 81, 82, 83, 84, 85 shown in FIG. 1 may represent an aggregatedload consisting of a plurality of individual loads, and the loads may bedistributed throughout the system such as on a system printed circuitboard. The loads, e.g. loads 81, 82, 83, 84B, 85B may include:microprocessors, semiconductor memory, information storage devices suchas electro-mechanical or solid state disk drives, lighting such as LEDbacklights or displays, either of which may include a controller,communication devices, e.g. as shown for the example of loads 84 and 85.In various embodiments, the loads 81, 82, 83, 84B, 85B includeintegrated circuits Each load or load group may further include one ormore power regulators to regulate power received from the powerconverter 60. Even a single load may present widely varying currentdemands from the power system 10, a microprocessor transitioning throughvarious computational states. Naturally, the aggregate of the loadsconnected to the output of bus converter 60 under normal operatingconditions may present even greater variations in the time-varying loadcurrent, IL, drawn from the converter 60, under essentially constantoutput voltage, V_(OUT), conditions. In some embodiments, the systemprinted circuit board may include the loads 81, 82, 83, 84B, 85B, theregulators 84A and 85A, the energy storage devices (e.g., capacitors 71,72, 73, 74, 75), the bus converter 60, and/or a power bus adapted toconnected the output of the bus converter 60 to the energy storagedevices and the regulators 84A and 85A. In some embodiments, any or allof the components, such as the regulators 84A and 85A, can be mounted tothe system circuit board. In some embodiments, various of the components(e.g., the bus converter 60 and control circuitry) may be packagedtogether as a self-contained assembly, such as an assembly adapted to beinstalled as a unit.

Referring to FIG. 3, a low frequency model 60A of the bus converter isshown including a controlled voltage source 150; an output resistance160 having a resistance, R_(O); and an input current sink 160.Resistance 160 in the simplified model 60A of FIG. 3 represents theequivalent output resistance of the bus converter. In an ideal DCtransformer, the output resistance, R_(O), may be zero. The controlledvoltage source 150 produces a voltage, V_(S), which is a function of thetransformation ratio, K, and the input voltage, V_(IN), to theconverter. Similarly, the controlled current sink 160, sinks a current,Is, which is a function of the transformation ratio, K, and the outputcurrent, I_(O) or I_(OUT), drawn from the converter. The transformationratio, K, of the converter depends upon the converter topology andconfiguration, e.g. transformer turns ratio (e.g., a ratio of a firstnumber of turns of a winding of input circuitry to a second number ofturns of a winding of output circuitry), full or half bridge drive orrectification circuitry, number of cells, series connection of input andoutput, etc.

The magnitude of V_(S) may deviate from the ideal value as a function ofload, temperature, and other factors in practical converterimplementations. Similarly, the input current, I_(IN), may includenon-linear components and load independent currents, e.g. the quiescentcurrent that a converter may draw from the source to power controlcircuitry, and other loss elements as a function of the operating pointof the converter, causing I_(IN) to depart from the ideal value, Is. Theoutput resistance, R_(O), may also vary as a function of the operatingpoint of the converter, including its temperature, etc. However, themodel shown in FIG. 3, generally provides a good first orderapproximation. In practical applications, it is generally desirable tominimize the magnitude of output resistance, R_(O), of the bus converterto limit power dissipation in the converter and maintain the outputvoltage with load changes, i.e. reduce the effects of R_(O). However,because the output voltage, V_(O), output current, I_(O), and thus inputcurrent, I_(IN), are affected by the magnitude of output resistance,R_(O), provision of a finite value of output resistance, R_(O), may bebeneficial to equalize load sharing among an array of power suppliesconnected in parallel (“droop compensation”).

In a practical switching converters, i.e., switches used in theconverter have parasitic capacitances and inductances, power may bedissipated in a switch when the switch is being turned ON, which iscalled “switching loss.” Capacitances, both parasitic and lumped, acrossa switch if not discharged before the switch is turned ON may be a majorcontributor to switching loss. One way to reduce switching loss in aswitching power converter (e.g., a buck, a boost, a buck-boost, or afixed ratio switching power converter) uses an inductive component tofully or partially charge and discharge the capacitances associated witha switch before turning it ON to achieve full or partial zero voltageswitching (“ZVS”) during an energy recycling interval (“ERI”) (which mayalso be called a “ZVS” interval). ZVS ideally causes the voltage acrossthe switch to decline to zero volts, essentially eliminating switchinglosses associated with the capacitive discharge of the switch; however,any significant reduction, e.g. by 50 percent, 80 percent, 90 percent,or more from the peak voltage across the switch, respectively reducesthe switching losses during turn ON by 75 percent, 96 percent, 99percent, or more. However, it may be difficult to control switches toturn ON or OFF at times when the voltage across the switch is at zerovolts or a minimum voltage due to a variety of factors, including fastvoltage or current transitions, very small signals, propagation delays,and noise in the converter. Turning switches ON and OFF at times whenzero or minimal current is flowing through the switch, called zerocurrent switching (“ZCS”), can also reduce losses and reduce noise.However, ZCS operation of switches also may be difficult for the samereasons as with ZVS.

Besides difficulty in ZVS and ZCS operations, it may also be difficultto sense the output current for a converter without dissipatingexcessive power. Typically, a sense resistor is connected in series withthe load at the output side of the converter and the voltage across thesense resistor is sensed to monitor the output current. A trade-offbetween the need for sufficient signal voltage across the resistor, e.g.relative to noise, and power dissipation in the resistor, may lead tosignificant power consumption in the resistor, impacting converterefficiency.

One DC Transformer topology, called a Sine Amplitude Converter (“SAC”),is described in Vinciarelli, Factorized Power Architecture with Point ofLoad Sine Amplitude Converters, U.S. Pat. No. 6,930,893 issued Aug. 16,2005; and in Vinciarelli, Point of Load Sine Amplitude Converters andMethods, U.S. Pat. No. 7,145,786 issued on Dec. 5, 2006, each assignedto VLT, Inc. and incorporated herein by reference in their entirety (the“SAC Patents”). The SAC DC transformer topology generally provides forvery low output resistance, a transformation ratio, K, which isessentially constant over a wide operating range, provides ZVS of allpower switches, provides ZCS of the secondary switches and partial ZCSof the primary switches (limited by magnetizing current in thetransformer), and non-dissipative means for sensing the output current.An non-isolated variant of the bus converter topology havingseries-connected input and output circuits is described in Vinciarelliet al., Power Distribution Architecture With Series-Connected BusConverter, U.S. application Ser. No. 13/933,252, filed Jul. 2, 2013assigned to VLT, Inc. and incorporated herein by reference in itsentirety.

Absent implementation of in-rush current limiting, at start-up the busconverter will attempt to establish the output voltage, V_(O), at itsoutput which is connected to the load, loads 81 through 85 (FIG. 1) andthe capacitances 71 through 75 (FIG. 1) within the bandwidth of theconverter. In some bus converters, such as a SAC operating over 1 MHz,the output may attempt to slew in under a microsecond. Because of thevery low output resistance, R_(O), and the capacitances 71-75 (FIG. 1)connected to the output of the bus converter 60 will generally bedischarged to a voltage, that is lower than the nominal output voltage,V_(O) or V_(OUT), (typically zero Volts on start-up); the converteroutput current, I_(O) or I_(OUT), (and consequently the input current,I_(IN)) at start-up may be very high. This current surge may be called“in-rush” current in fixed ratio converters. In many cases, the in-rushcurrent required to charge the capacitances 71-75, may exceed thecurrent carrying capacity and/or dissipation limits of the switchingelements, e.g. MOSFETs, and other components, e.g. inductor windings,magnetically permeable cores, in the converter 60 and at the source 50(e.g. sags, current limiting, tripping of circuit breakers and fuses),potentially and may negatively impact operation of other systems thatare connected to the same input source 50. Some loads may draw largerthan normal currents when operated at voltages below their normaloperating voltage, which may further exacerbate the in-rush effectduring startup.

Various approaches (not shown) to controlling the input current duringstartup may include using a fixed or current-dependent resistance device(such as Negative-Temperature-Coefficient (NTC) thermistor) or aswitching regulator, e.g. a buck regulator, in series with the busconverter input, i.e. between the source 50 and the bus converter input,to slowly ramp the input voltage, however, the persistent presence ofthe series resistance of the above approaches can cause loss ofefficiency during steady-state operation. As described below, increasingthe bus converter output resistance during startup, e.g. as a functionof input current, may effectively control in-rush current withoutincurring the persistent efficiency penalty of the above approaches.Modulating the equivalent output resistance of a power converter, forexample a SAC, by varying the ON resistance of the primary switches inthe converter, as a means, e.g. of “soft-starting” the converter,performing output current limiting or improving current sharing betweenconverters in a power sharing array, is taught in Vinciarelli, OutputResistance Modulation in Power Converters, U.S. Pat. No. 6,934,166,issued on Aug. 23, 2005 (the “166 Patent”), assigned to VLT, Inc. andincorporated herein by reference in its entirety.

As shown in FIG. 1, the loads may preferably include a control input,e.g. control inputs, e.g. 90 a, 90 b, 90 c, 90 d, 90 e for individualload elements 81, 82, 83, 84, and 85, which may be used to influence theamount of current drawn by each load during a start-up phase. Forexample, control inputs 90 a through 90 e may receive a respectiveenable/disable signal in response to which the respective load elementor group functionally connects or disconnects from the output of powerconverter 60. Functional disconnection of the load may include a standbystate or standby mode in which current draw from the power converter 60is reduced or limited, preferably to a minimal level, such as 20 percentor less of the normal level. For example, the enable/disable controlsignal may cause a microprocessor, ASIC, memory, or other electronicload to enter or remain in an inactive state, e.g. a sleep mode, inwhich the current drawn from the power system may be reduced to a muchlower or negligible level compared with during its respective normaloperational state, e.g. by using a lower clock frequency or keepingnon-critical operations or portions of the circuit powered off, ordisabling read/write operations, etc. During system start-up, thedisable signal may be used to delay an initialization process until theenable signal is present. In other types of circuits, the enable/disablesignal may disable transmitter circuitry, displays drivers, lightingcontrollers, disk drive motors and circuitry and so on. The aggregateload or each individual load element may be considered disabled when thecurrent drawn is limited to 20 percent or less of its steady state levelor of its peak level during normal operation.

Referring to FIGS. 2A through 2F, waveforms for an in-rush limited busconverter 60 used with start-up control in power system 10 include: aconverter enable signal (FIG. 2A); converter input current, I_(IN),(FIG. 2B); converter output voltage, V_(OUT), (FIG. 2C); a load enablesignal (FIG. 2D); load current (shown scaled to the input side using thetransformation ratio, I_(LOAD)×K) (FIG. 2E); and a switch drive level(FIG. 2F) during the startup phase, i.e. from time T₀ to time T_(SS),and the subsequent normal or steady-state operation, i.e. after timeT_(SS). The startup phase may be defined as beginning when twoconditions are met (at time T₀ in FIGS. 2A-2F: (1) the bus converter 60receives power from the source and (2) the power converter 60 is enabledto process power from its input for delivery to its output, e.g. using aconverter enable signal such as shown in FIG. 2A. At time T₀, the busconverter 60 receiving the Converter Enable signal (FIG. 2A; 204, FIG.4) begins converting power from its input to its output in a start-upphase that includes in-rush current limiting. It is assumed that powerfrom the source 50 is available at the input of the converter 60 beforethe Converter Enable signal is asserted at time, T₀. Referring to thewaveforms for input current (FIG. 2B) and output voltage (FIG. 2C), thebus converter 60 may operate the power switches of the converter tolimit the input and output current, forcing the output voltage, V_(OUT),to increase gradually from zero until reaching the steady-stateoperating value, V_(O)≈K*V_(IN), at time T_(S). As discussed in moredetail below in connection with FIGS. 4 and 5, selected power switchesin the converter may be operated in a high-impedance state duringstart-up to increase the output resistance R_(O) during start-up.

At time T_(S), the start-up phase may end and normal or steady-stateoperation may commence. As shown in FIG. 2D the Load Enable signal maybe generated a short time after the converter completes its start-upphase. Alternatively the Load Enable signal may be generated coincidentwith or even just before the converter finishes start-up, depending onthe system configuration, bus converter implementation, and loadbehavior. As shown in FIG. 2E the load current ramps up to its normalsteady state level just after the Load Enable signal. In steady-stateoperation the power converter output voltage, V_(O), is maintainedwithin its normal steady-state operating range, V_(O)≈K*V_(IN), (FIG.2C), the output current, I_(O) varies within an expected range (notshown) and the power converter input current, I_(IN), (FIG. 2B) variesaccordingly within its expected range (not shown).

FIG. 4 shows a bus converter 60 based on the SAC topology including anideal transformer T1 209, a resonant circuit including Cr 212 and Lr 213connected in series with the primary winding 214 of transformer 209,primary switches S1 210, S2 211 connected to drive the transformer andresonant circuit; secondary switches SR1 216, SR2 217, connected to thesecondary winding 215 as synchronous rectifiers, control circuitry 200,including switch controller 201, switch driver circuitry 202, input 204for receiving a Converter Enable signal and input 203 for sensing inputcurrent. Detailed operation of the SAC topology is described in the SACPatents and the basic principles of adjusting the switch drive tomodulate the output resistance of the bus converter for current limitingare described in the 166 Patent, all of which are incorporated byreference.

Referring to FIG. 5, an overview of the operation of the system 10 ofFIG. 1 during start-up is shown. Upon start-up, e.g. upon receivingpower after an off period or following a reset), the load may bedisabled by issuing or maintaining a Load Disable signal (FIG. 2D; 90 a,90 b, 90 c, 90 d, 90 e, FIG. 1) as shown in block 411, and thecontroller 200 (FIG. 4) in bus converter 60 may wait to receive aconverter enable signal (FIG. 2A; input 204, FIG. 4). The system mayissue a Convert Enable signal to the bus converter 60 at time T₀ (FIG.2A), which controller 200 receives on input 204 (FIG. 4) and accordinglyinitiates power conversion operating cycles in the converter, i.e.issuing control signals (not shown) for turning the power switches, e.g.S1, S2, SR1, SR2, ON and OFF as shown in block 412 entitled “startswitch control sequence” (FIG. 5). The switch driver 202 (FIG. 4)translates the switch control signals, e.g. logic control signals, intothe appropriate switch drive signals for operating the switches, e.g.the appropriate gate drive voltages for MOSFETs, etc. The switch driver202 gradually ramps up the switch drive level from a minimum to the fulldrive level, i.e. the maximum, Max, using the conceptual start-upsequence shown in blocks 413, 414, and 415 in FIG. 5. As shown, theswitch driver 202 senses and compares the input current (FIG. 2B) to aninrush limit (413, FIG. 5; input 203, FIG. 4) which as shown in FIG. 2Bis less than the typical maximum load current. While the input currentis less than the in-rush limit, the switch drive 202 gradually increasesthe switch drive level (414, FIG. 5; FIG. 2F) until the full switchdrive level is attained. If at any point during the ramp-up, the inputcurrent equals or exceeds the in-rush limit (the “No” condition, 413,FIG. 5), the switch driver waits for the input current to once againdrop below the in-rush limit before proceeding. An example of thisfunctionality is illustrated in FIGS. 2B and 2F, where at time T_(X),the input current (FIG. 2B) exceeds the in-rush limit causing the switchdrive level (FIG. 2F) to stop increasing. As shown from time T_(X) totime T_(Y), the input current (FIG. 2B) remains above the in-rush limitcausing the switch drive level (FIG. 2F) to remain constant. At timeT_(Y), the input current once again drops below the in-rush threshold(FIG. 2B) and the switch drive level again starts to increase toward thefull switch drive level (Max) (FIG. 2F). At time T_(S), the switch drivelevel has increased to its maximum, e.g. the full drive level. As shownin FIGS. 2B and 2C respectively, the input current, I_(IN), has droppedsignificantly, e.g. to zero or its quiescent or no-load level, and theoutput voltage has increased to K*V_(IN), indicating that thecapacitances, e.g. 71, 72, 73, 74, 75 (FIG. 1) connected to the outputof the bus converter 60 have been fully charged. As shown in functionblock 416 (FIG. 5) the system may enable the load using the loadenable/disable signal either essentially coincident with, or shortlyafter, the switch drive level reaches the maximum at time T_(S) in FIG.2F, i.e. the “Yes” condition, in decision block 415 (FIG. 5). The loadenable signal is shown issuing at time T_(SS) in FIG. 2D a short timeafter T_(S). The start-up sequence may end after the load is enabled asshown in block 417 (FIG. 5).

To summarize, the switch driver 202 may adjust the switch drive levelbased upon a comparison of the input current to desired maximum in-rushcurrent level during start-up, thus controlling the rate at which theswitch drive level rises. As the switch drive level increases, theswitches may pass through a linear region of operation, for example nearthe threshold voltage of MOSFET switches. The ON resistance of theswitches, and the equivalent output resistance of the converter, R_(O),will thus vary smoothly from a relatively high value to a relatively lowvalue, reducing the peak currents that would otherwise flow as theconverter output filter capacitors (e.g. capacitors 71 through 75)charge to their final values (V_(O)), and the rate at which the outputvoltage rises to its final value, V_(O)≈K*V_(IN) after the inputvoltage, V_(IN), is applied to the power converter 60. It should benoted that the input current is essentially a function of the outputcurrent as shown and described in connection with FIG. 3 above, whichprovides a good measure of the output current without requiring signalsto cross the isolation boundary in isolated bus converters. Furthermore,as described below in connection with FIG. 6, a measure of the inputcurrent may be derived from the voltage across the resonant capacitor Cr212 (FIGS. 4, 6) as described in the SAC patents. After start-up insteady-state operation, the switch drive levels are maintained tominimize the ON resistance of the switches and the output resistance,R_(O), (FIG. 3) of the converter. The bus converter 60 may convenientlygenerate the load enable/disable signal(s) 90 a through 90 e (FIG. 1,FIG. 2D) as described below in connection with FIG. 6, e.g. as theswitch drive level (which may be generated internally to the busconverter) reaches its maximum. Alternatively, a supervisory controller(not shown) may control the loads with status information provided bythe bus converter, or by monitoring the input current or output currentof the converter, which with all loads disabled will drop to a minimallevel after the capacitances are charged during start-up. As describedabove, the enable/disable signal may disable the active load(s) duringstart-up eliminating load-related contributions to the input currentsurge.

Referring to FIG. 6, a modified bus converter 60B is shown with thecontroller 200B having input 203 connected to node 223 to sense thevoltage, V_(Cr), across the resonant capacitor, Cr 212; an output 209for issuing a load enable signal; and an output 221 for issuing an overcurrent signal. As noted above the voltage, V_(Cr), across the resonantcapacitor, Cr 212, may be used as a measure of the input or outputcurrent. The potential of the left end of capacitor Cr 212 connected tonode 222 (between the two input capacitors, C_(IN1) 218, C_(IN2) 219) isessentially constant at V_(IN)/2, allowing the controller toconveniently sense the voltage, VCr, using a current sense input 203 asshown in FIG. 6 connected to node 223. The controller 200B may comparethe voltage at node 223 to an internal V_(IN)/2 reference oralternatively use a differential input connected to sense both ends ofthe resonant capacitor, nodes 222 and 223. As described above, thecontroller 200B may issue the Load Enable signal via output 209 (416,FIG. 5) essentially coincident with, or shortly after, the switch drivelevel reaches the maximum (415, FIG. 5).

The current monitoring feature of the controller 200B also may be usedafter start-up, e.g. during steady state operation, to monitor theoutput current, e.g. for over-current and transient current conditionsand issue one or more overcurrent signals, e.g. via output 221, e.g.indicating that the output current level, or the rate of change of theoutput current, has met or exceeded one or more predeterminedthresholds. Such a signal may be received by one or more of the loads81, 82, 83, 84, 85 (FIG. 1) via the respective inputs 90 a, 90 b, 90 c,90 d, 90 e (in addition to the Load Enable signals) to either reduce theinstantaneous or average current drawn by the load to stay withinpredefined limits, compensate for an ensuing voltage perturbation in theoutput voltage, e.g. due to a transient current meeting or exceedingpredetermined criteria, or both. In some embodiments, the controller200B may include an input used to determine the input and/or outputcurrent and disable one or more of the loads 81, 82, 83, 84B, 85B usingthe control signal when the control circuit determines the input oroutput meets the predetermined criteria (e.g., a predetermined change inthe current, a predetermined rate of change in the current, or apredetermined threshold level of the current). For example, a largesemiconductor load, such as a CPU, graphics or mathematicalco-processor, an ASIC, etc. or large systems, e.g. an automotive system,battery charger, etc. may have the capacity to adjust load demands, e.g.by placing some non-critical circuitry or systems in standby or reducepower operation, or make internal adjustments to accommodate a voltagetransient for which adequate notice is provided. The controller 200B mayissue a binary signal indicating an over-load, or load transientcondition, or may include information identifying the condition and/orthe relative magnitude of the condition. For example, the output 221 maysink a current proportional to the load, or use any of a number ofserial communication protocols to indicate the nature and optionally themagnitude of the event. The converter 60B may further adjust the switchdrive levels of the power switching elements during steady-stateoperation in the event of an over-current condition.

The example of FIGS. 1-6 are some of the many possible configurations inwhich startup current surge may be controlled by combining switchresistance modulation with active load control. The power converter 60may comprise any of a variety of isolated and non-isolated convertertopologies. For example, many known DC transformer technologies may beconfigured to operate with resistance modulation. Resistance modulationmay be used in DC transformers having K factors less than, equal to, orgreater than one. Similarly, resistance modulation for surge control maybe incorporated into other known isolated and non-isolated fixed ratioconverter topologies resonant and hard switched converters and switchedcapacitor converter topologies. Additionally, alternate methods ofsensing input or output current in fixed ratio converters may be used.For example, current sensing as a function of the voltage drop acrossthe bus converter is described in FIG. 7 of Vinciarelli, AdaptiveControl of Resonant Power Converters, U.S. patent application Ser. No.15/715,818, filed Sep. 26, 2017 (the “ASAC patent”) assigned to VLT,Inc. and incorporated here in its entirety by reference. In someimplementations, the input circuitry and at least a portion of theoutput circuitry are galvanically connected in series across the sourcefor at least a portion of the converter operating cycle, such that anabsolute value of the input voltage, V_(IN), applied to the inputcircuit is approximately equal to the absolute value of the DC sourcevoltage V_(S) minus a number, N, times the absolute value of the outputvoltage, V_(OUT), where N is at least 1. In some implementations,

Adjusting MOSFET gate drive voltage within the linear region may provideorders of magnitude of channel resistance variation, thereby providing awide range of variation in equivalent output resistance. Other methodsmay also produce smaller, but possibly useful, variations in equivalentoutput resistance, R_(O), during start-up. For example, configuring aSAC controller to increase or decrease the duration of the resonantinterval may produce a variation in equivalent output resistance, R_(O),as described in Vinciarelli, Clamped Capacitor Resonant Power Converter,U.S. Pat. No. 9,325,247, issued on Apr. 26, 2016 (the “CSAC patent”)assigned to VLT, Inc. and incorporated here in its entirety byreference; and in the ASAC patent.

Additionally, the above methods may be combined with configurable arraysof bus converters, whose system transformation ratio may be adaptivelyconfigured to perform output voltage scaling, e.g. as described inVinciarelli, Adaptively Configured Voltage Transformation Module Array,U.S. Pat. No. 7,170,764, Issued Jan. 30, 2007; in Vinciarelli,Adaptively Configured and Autoranging Voltage Transformation ModuleArrays, U.S. Pat. No. 7,212,419, Issued May 1, 2007; in AdaptivelyConfigured and Autoranging Voltage Transformation Module Arrays, U.S.Pat. No. 7,420,825, Issued Sep. 2, 2008; in Vinciarelli, AdaptivelyConfigured and Autoranging Voltage Transformation Module Array, U.S.Pat. No. 7,423,892, Issued Sep. 9, 2008; in Adaptively Configured andAutoranging Power Converter Arrays, U.S. Pat. No. 7,782,639, Issued Aug.24, 2010, each assigned to VLT, Inc. and each incorporated herein byreference in its entirety. By connecting converter inputs in seriesand/or parallel and connecting converter outputs in series and/orparallel, for any given input voltage, the output voltage of an array ofconverters may be varied over a range. For example, if the modules inthe array are DC Transformers (e.g. SACs) and the array is configuredwith all of the converter inputs in series and all of the converteroutputs in parallel, the transformation ratio will be at a minimum andthe output voltage of the array will be at a minimum value.Alternatively, if all of the converter inputs are connected in paralleland all of the converter outputs are connected in series, thetransformation ratio and the output voltage of the array will be at amaximum. Other configurations of inputs and outputs will produce arrayoutput voltages that are between those minimum and maximum values. Thismethod of configuring DC Transformers is effectively a way of modifyingthe overall K factor of the array. Array configuration may be used incombination with resistance modulation to effect a greater range ofeffective output resistance modulation and/or to improve power sharingamong converters. If there is sufficient granularity in the array, andthe change in output voltage caused by array reconfiguration can be madesufficiently small, array reconfiguration alone may be sufficient tocontrol the input current surge to an acceptable level.

For the purposes of the present disclosure, values may be considered“equal,” “substantially the same,” “substantially equal,” “essentiallyequal,” etc. when the values are exactly equal to or nearly equal to oneanother.

A number of embodiments of the invention have been described. Theforegoing description of embodiments of the disclosure has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the disclosure to the precise formdisclosed, and modifications and variations are possible in light of theabove teachings or may be acquired from practice of the disclosure. Theembodiments were chosen and described in order to explain the principlesof the disclosure and its practical application to enable one skilled inthe art to utilize the disclosure in various embodiments and withvarious modifications as are suited to the particular use contemplated.Therefore, it will be understood that various modifications may be madewithout departing from the spirit and scope of the invention. Forexample, the in-rush current may be limited, e.g. during start-up, as afunction power dissipation in one or more of the switching elements,e.g. by monitoring the current and voltage across one or more of theswitches, or monitoring any combination of the following: input current,resonant capacitor voltage, switch voltage, voltage differential acrossthe bus converter, etc.

Furthermore, it should be noted that although the disclosure providedherein may describe a specific order of method steps, it is understoodthat the order of these steps may differ from what is described, thattwo or more steps may be performed concurrently or with partialconcurrence, and that the functions of one or more steps may implementedin circuitry. It is understood that all such variations are within thescope of the disclosure.

What is claimed is:
 1. An apparatus for delivering power from an inputsource at a source voltage, V_(S), to a load at a load voltage, V_(L),the apparatus comprising: a bus converter having a converter input, aconverter output, a plurality of power switching elements, and a switchcontroller adapted to operate the power switching elements in a seriesof converter operating cycles during start-up and steady-state phases ofoperation to convert power received from the converter input at an inputvoltage, V_(IN), and input current, I_(IN), for delivery to theconverter output at an output voltage, V_(OUT), and an output current,I_(OUT), using a transformation ratio, K, where K=V_(OUT)/V_(IN), thatis essentially constant; the bus converter being adapted for operationin the start-up phase to turn one or more of the power switchingelements ON in a high resistance state to increase an equivalent seriesresistance of the bus converter and for operation in the steady-statephase to turn one or more of the power switching elements ON in a lowresistance state to decrease the equivalent series resistance of the busconverter; and a control circuit having an output to provide a controlsignal to disable the load during the start-up phase and to enable theload during the steady-state phase; wherein the switch controller isadapted to begin operation of the bus converter in the start-up phaseafter the switch controller receives power and a converter enablesignal; wherein the switch controller is adapted to use a switch controlsequence to adjust a drive level for one or more of the power switchingelements within a range during the start-up phase from an initial valuefor turning the one or more of the power switching elements ON with ahighest switch resistance to a final value for turning the one or moreof the power switching elements ON with a lowest switch resistance;wherein the switch control sequence includes determining whether acurrent in the converter meets or exceeds a predetermined limit;incrementing the drive level if the current does not meet or exceed thelimit and repeating the determining and incrementing until a maximumdrive level is reached, and exiting the start-up phase after the maximumdrive level is reached.
 2. The apparatus of claim 1, wherein the switchcontroller is further adapted to operate the bus converter in thestart-up phase when the output voltage, V_(OUT), is below a firstthreshold and in the steady-state phase when the output voltage,V_(OUT), is above a second threshold.
 3. The apparatus of claim 1,wherein the switch controller is further adapted to provide a signalindicating that the current drawn from the converter output meets orexceeds a predetermined criteria during steady-state operation.
 4. Theapparatus of claim 3, wherein the predetermined criteria includes apredetermined change in the current.
 5. The apparatus of claim 3,wherein the predetermined criteria includes a predetermined rate ofchange in the current.
 6. The apparatus of claim 3, wherein thepredetermined criteria includes a predetermined threshold level of thecurrent.
 7. The apparatus of claim 1, wherein the bus converter and thecontrol circuit are packaged together as a self-contained assemblyadapted to be installed as a unit.
 8. The apparatus of claim 1, furthercomprising: a system circuit board including the load and a power busadapted to connect to the bus converter output; one or more energystorage devices connected to the power bus converter; one or moreregulators, each having a regulator input connected to receive powerfrom the power bus, a regulator output connected to deliver power to theload, and regulation circuitry adapted to convert power received fromthe regulator input for controlled delivery to the load via theregulator output; and wherein the load comprises electronic circuitryincluding an integrated circuit.
 9. The apparatus of claim 8, whereinthe bus converter and the one or more regulators are assembled to thesystem circuit board, and the one or more regulators are disposed at adistance from the bus converter.
 10. The apparatus of claim 1, whereinthe bus converter further comprises input circuitry and outputcircuitry, and at least a portion of the input circuitry and outputcircuitry are galvanically connected in series across the source for atleast a portion of the converter operating cycle, such that an absolutevalue of the input voltage, V_(IN), applied to the input circuit isapproximately equal to the absolute value of the source voltage, V_(S),minus a number, N, times the absolute value of the output voltage,V_(OUT), where N is at least
 1. 11. The apparatus of claim 1, whereinthe control circuit further is further adapted to disable the load usingthe control signal when the control circuit determines the outputvoltage, V_(OUT), is below a predetermined threshold.
 12. The apparatusof claim 11, wherein the predetermined threshold is a function of theinput voltage, V_(IN).
 13. The apparatus of claim 12, wherein thedetermining is a function of the input voltage, V_(IN), or outputvoltage, V_(OUT), adjusted by the transformation ratio K.
 14. Theapparatus of claim 12, wherein the determining is a function of adifference between the input voltage, V_(IN), and output voltage,V_(OUT), one of which is adjusted by the transformation ratio K.
 15. Theapparatus of claim 1, wherein the bus converter comprises an inductivecomponent and a current flowing in the inductive component charges anddischarges capacitances in the bus converter reducing a voltage acrossone or more one of the power switching elements prior to turning ON saidone or more power switching element.
 16. The apparatus of claim 1,wherein the bus converter comprises a switched capacitor converter. 17.The apparatus of claim 1, wherein the bus converter further comprises atransformer, input circuitry including one or more primary powerswitching elements connected to drive the transformer and outputcircuitry connected to receive power from the transformer, a resonantcircuit which may include the transformer, the resonant circuit having acharacteristic resonant frequency and period, the one or more primarypower switching elements are connected to drive the resonant circuit,and each converter operating cycle is characterized by two powertransfer intervals of essentially equal duration, each interval having aduration less than the characteristics resonant period, during which oneor more of the primary power switching elements is ON and power istransferred from the input circuitry to the output circuitry via thetransformer.
 18. The apparatus of claim 17, wherein the switchcontroller is further adapted to monitor a voltage across a resonantcapacitor for determining whether a current in the converter meets orexceeds the predetermined limit.
 19. An apparatus for delivering powerfrom an input source at a source voltage, V_(S), to a load at a loadvoltage, V_(L), the apparatus comprising: a bus converter having aconverter input, a converter output, a plurality of power switchingelements, and a switch controller adapted to operate the power switchingelements in a series of converter operating cycles during which powerreceived from the converter input at an input voltage, V_(IN), and inputcurrent, I_(IN), is converted for delivery to the converter output at anoutput voltage, V_(OUT), and an output current, I_(OUT), via the powerswitching elements during a start-up phase of operation and during asteady-state phase of operation; the bus converter being operative inthe steady-state phase of operation to convert power using atransformation ratio, K, where K=V_(OUT)/V_(IN), that is essentiallyconstant; the bus converter being operative in the start-up phase ofoperation to limit a current flowing through the power switchingelements to a predetermined in-rush limit, the in-rush limit being lessthan a level of current permitted during the steady-state phase; theswitch controller being adapted to begin operation of the bus converterin the start-up phase after the switch controller receives power and aconverter enable signal; the switch controller being adapted to use aswitch control sequence to adjust a drive level for one or more of thepower switching elements within a range during the start-up phase froman initial value for turning the one or more of the power switchingelements ON with a highest switch resistance to a final value forturning the one or more of the power switching elements ON with a lowestswitch resistance; the switch control sequence including determiningwhether a current in the converter meets predetermined criteria;incrementing the drive level if the current does meet the criteria andrepeating the determining and incrementing until a maximum drive levelis reached, and exiting the start-up phase after the maximum drive levelis reached; and a control circuit having an output to provide a controlsignal to disable the load during the start-up phase of operation and toenable the load during the steady-state phase of operation.
 20. Theapparatus of claim 19, wherein the switch controller operates the busconverter in the start-up phase when the output voltage, V_(OUT), isbelow a first threshold and in the steady-state phase when the outputvoltage, V_(OUT), is above a second threshold.
 21. The apparatus ofclaim 19, wherein the switch controller sends a signal to the controlcircuit after the final value is reached and the control circuit isincluded in the bus converter.
 22. The apparatus of claim 19, whereinthe switch controller further comprises an input connected to monitor avoltage across a resonant capacitor and an output adapted to provide asignal indicating that a current drawn from the converter output meetsor exceeds predetermined criteria during steady-state operation, thepredetermined criteria including a predetermined change in the current,a predetermined rate of change in the current, or a predeterminedthreshold level of the current.
 23. The apparatus of claim 19, furthercomprising: one or more energy storage devices connected to the busconverter output; and one or more regulators, each having a regulatorinput connected to receive power from the bus converter output, aregulator output, and regulation circuitry adapted to convert powerreceived from the regulator input for controlled delivery to the loadvia the regulator output.
 24. The apparatus of claim 23, wherein the oneor more regulators each further comprises a control input connected toreceive the control signal and is adapted to restrict power delivery tothe regulator output during the start-up phase.
 25. The apparatus ofclaim 24, wherein the one or more regulators comprise switching voltageregulators.
 26. The apparatus of claim 25, further comprising: a systemcircuit board including the load and a power bus adapted to connect thebus converter output to the one or more energy storage devices and theone or more regulators; and wherein the load comprises electroniccircuitry including an integrated circuit; and wherein the one or moreregulators are mounted to the system circuit board.
 27. The apparatus ofclaim 24, wherein the load further comprises electronic circuitryconnected to receive power from the regulator output of the one or moreregulators, a control input connected to receive the control signal isadapted to restrict the power it draws from the regulator output duringthe start-up phase.
 28. The apparatus of claim 27, wherein the one ormore regulators comprise switching voltage regulators.
 29. The apparatusof claim 27, further comprising: a system circuit board including theload and a power bus adapted to connect the bus converter output to theone or more energy storage devices and the one or more regulators; andwherein the load comprises electronic circuitry including an integratedcircuit; and wherein the one or more regulators are mounted to thesystem circuit board.
 30. The apparatus of claim 29, wherein the busconverter comprises an inductive component and a current flowing in theinductive component charges and discharges capacitances in the busconverter reducing a voltage across one or more one of the powerswitching elements prior to turning ON said one or more power switchingelement.
 31. The apparatus of claim 21, wherein the bus converter andthe control circuit are packaged together as a self-contained assemblyadapted to be installed as a unit.
 32. The apparatus of claim 19,further comprising: a system circuit board including the load and apower bus adapted to connect to the bus converter output; one or moreenergy storage devices connected to the power bus converter; one or moreregulators, each having a regulator input connected to receive powerfrom the power bus, a regulator output connected to deliver power to theload, and regulation circuitry adapted to convert power received fromthe regulator input for controlled delivery to the load via theregulator output; and wherein the load comprises electronic circuitryincluding an integrated circuit.
 33. The apparatus of claim 32, whereinthe bus converter and the one or more regulators are assembled to thesystem circuit board, and the one or more regulators are disposed at adistance from the bus converter.
 34. The apparatus of claim 19, whereinthe bus converter further comprises a transformer, input circuitryincluding one or more primary power switching elements connected todrive the transformer and output circuitry connected to receive powerfrom the transformer, a resonant circuit which may include thetransformer, the resonant circuit having a characteristic resonantfrequency and period, the one or more primary power switching elementsare connected to drive the resonant circuit, and each converteroperating cycle being characterized by two power transfer intervals ofessentially equal duration each interval having a duration less than thecharacteristics resonant period, during which one or more of the primarypower switching elements is ON and power is transferred from the inputcircuitry to the output circuitry via the transformer.
 35. The apparatusof claim 19, wherein the bus converter further comprises a transformer,input circuitry including one or more primary power switching elementsconnected to drive the transformer and output circuitry connected toreceive power from the transformer, and wherein the bus converter outputis galvanically isolated from the bus converter input.
 36. The apparatusof claim 19, wherein the bus converter further comprises a transformer,input circuitry including one or more primary power switching elementsconnected to drive the transformer and output circuitry connected toreceive power from the transformer, and wherein the bus converter outputis galvanically connected to the bus converter input.
 37. The apparatusof claim 19, wherein the bus converter further comprises a transformer,input circuitry including one or more primary power switching elementsconnected to drive the transformer and output circuitry connected toreceive power from the transformer, and wherein the input circuitrycomprises a winding having a first number of turns, the output circuitrycomprises a winding having a second number of turns, and thetransformation ratio is a function of a ratio of the first number ofturns to the second number of turns.
 38. The apparatus of claim 37,wherein the input circuitry and at least a portion of the outputcircuitry are galvanically connected in series across the source for atleast a portion of the converter operating cycle, such that an absolutevalue of the input voltage, V_(IN), applied to the input circuit isapproximately equal to the absolute value of the source voltage, V_(S),minus a number, N, times the absolute value of the output voltage,V_(OUT), where N is at least
 1. 39. The apparatus of claim 19, whereinthe bus converter further comprises input circuitry and outputcircuitry, and at least a portion of the input circuitry and outputcircuitry are galvanically connected in series across the source for atleast a portion of the converter operating cycle, such that an absolutevalue of the input voltage, V_(IN), applied to the input circuit isapproximately equal to the absolute value of the source voltage, V_(S),minus a number, N, times the absolute value of the output voltage,V_(OUT), where N is at least
 1. 40. The apparatus of claim 19, whereinthe bus converter further comprises an inductive component and a currentflowing in the inductive component charges and discharges capacitancesin the bus converter and wherein a current through one or more one ofthe power switching elements is reduced prior to turning said one ormore power switching elements ON or OFF.
 41. The apparatus of claim 21,wherein the bus converter includes the control circuit.
 42. Theapparatus of claim 21, wherein the control circuit is further adapted todisable the load using the control signal when the control circuitdetermines the output voltage, V_(OUT), is below a predeterminedthreshold.
 43. The apparatus of claim 30, wherein the voltage acrosssaid one or more power switching elements is reduced to essentially zerovolts prior to turn ON.
 44. The apparatus of claim 23, wherein the busconverter comprises a switched capacitor converter.
 45. An apparatus fordelivering power from an input source at a source voltage, V_(S), to aload at a load voltage, V_(L), the apparatus comprising: a bus converterhaving a transformer, a converter input, a converter output, a pluralityof power switching elements, and a switch controller adapted to operatethe power switching elements in a series of converter operating cyclesto convert power received from the converter input at an input voltage,V_(IN), for delivery via the transformer to the converter output at anoutput voltage, V_(OUT); the bus converter being adapted to monitor avoltage across a resonant capacitor and to provide an output signalindicating that a current drawn from the converter output during asteady state phase meets or exceeds predetermined criteria.
 46. Theapparatus of claim 45 wherein: the bus converter is adapted foroperation in a steady-state phase to convert power using atransformation ratio, K, where K=V_(OUT)/V_(IN), that is essentiallyconstant and during which one or more of the power switching elementsare turned ON in a low resistance state to control the equivalent seriesresistance of the bus converter; the bus converter is further adaptedfor operation in a start-up phase to turn one or more of the powerswitching elements ON in a high resistance state to increase theequivalent series resistance of the bus converter; and the bus converterincludes an output to provide a control signal to disable the loadduring the start-up phase and to enabled the load during thesteady-state phase.
 47. The apparatus of claim 46 wherein: the busconverter is adapted to begin operation in the start-up phase afterreceiving power and a converter enable signal; the bus converter isadapted to use a switch control sequence to adjust a drive level for oneor more of the power switching elements within a range during thestart-up phase from an initial value for turning the one or more of thepower switching elements ON with a highest switch resistance to a finalvalue for turning the one or more of the power switching elements ONwith a lowest switch resistance; and the switch control sequenceincludes determining whether a current in the converter meets or exceedsa predetermined limit; incrementing the drive level if the current doesnot meet or exceed the limit and repeating the determining andincrementing until a maximum drive level is reached, and exiting thestart-up phase after the maximum drive level is reached.